A problem of relevance to this invention has been brought about by the habit of telephone equipment and other installers to add new load circuits to existing power supply circuits, such as in fuse panels and the like. In the past, telephone engineers in the central office listed each new addition in a log which continually apprised them of the load situation. However, deregulation, personnel cutbacks and increased admission of outside suppliers and equipment has brought about a decentralization which does no longer lend itself to the maintenance of reliable power distribution logs.
Also, standard fuse panels typically come with some twenty to forty output fuse positions, even when only a fraction thereof will use the entire capacity in a given situation. In that case, an installer often gains the erroneous impression that there is plenty of capacity left. He or she thus naturally will try to make use of the empty positions.
Little by little, the total load thus connected to the power supply circuitry would approach and eventually exceed rated load capacity. In this manner, a main fuse or other overload current interrupter would be caused to blow or trip, even though there is no overload condition in any one circuit supplied with power through that main fuse or overload current interrupter.
In consequence, an entire system or major parts thereof are suddenly and unexpectedly laid idle, even when there is nothing wrong with any load circuit or supply line in that system.
This can have disasterous consequences, such as when thousands of communication channels are suddenly interrupted, just because new equipment has been added to the system. However, that happens with increasing frequency, especially now that fiber optics long-distance channels are being installed all over the country. While this example is taken from the field of telephony, progressive overloading of power supply circuits through progressive addition of load circuits is also a problem in other areas.
In principle, an ammeter or similar measuring instrument could be employed to indicate to the installer or other concerned person when a safe limit has been reached. However, such instruments have in fact been disappearing from power supply panels, where they were used decades ago for other purposes, and it would not appear that expecting installers to continually read instruments would present a practical solution. Also, there is a difference between expecting an installer to read instruments on the one hand, and to provide positive alarms on the other.
In a different vein, light emitting diodes (LED's) have become increasingly popular as indicators, and there even is a type in which three different colors can be produced with two diodes. However, that as such does of course not solve the above mentioned problem.
There also has been a problem of obtaining a quick indication when either a main or input fuse or other overload current interrupter or an output fuse or further overload current interrupter of a power supply circuit has blown or tripped in response to one or more overload conditions.
A detailed inspection of the various fuses or other over-load current interrupters takes time and is subject to error by oversight. What is needed is a rapidly acting signaling system that differentiates between power interruption by a main fuse or overload current interrupter on the one hand, and the blowing or tripping of a fuse or overload current interrupter of lesser rank.
Moreover, there is a need for improved equipment that in effect pulls together the alarm conditions of different fuse panels or other circuits and provides a central indication of all alarms.
When monitoring fuses or other interrupters, a drawback also has been that prior-art monitoring equipment of an active type introduced reliability problems of its own. The need for passive monitoring systems thus has arisen.